Wireless communications systems are used in a variety of telecommunications systems, television, radio and other media systems, data communication networks, and other systems to convey information between remote points using wireless transmitters and wireless receivers. A transmitter is an electronic device which, usually with the aid of an antenna, propagates an electromagnetic signal such as radio, television, or other telecommunications. Transmitters often include signal amplifiers which receive a radio-frequency or other signal, amplify the signal by a predetermined gain, and communicate the amplified signal. On the other hand, a receiver is an electronic device which, also usually with the aid of an antenna, receives and processes a wireless electromagnetic signal. In certain instances, a transmitter and receiver may be combined into a single device called a transceiver.
Transmitters, receivers, and transceivers often include components known as oscillators. An oscillator may serve many functions in a transmitter, receiver, and/or transceiver, including generating local oscillator signal (usually in a radio-frequency range) for upconverting baseband signals onto a radio-frequency (RF) carrier and performing modulation for transmission of signals, and/or for downconverting RF signals to baseband signals and performing demodulation of received signals.
To achieve desired functionality, such oscillators must often have designs that produce precise operating characteristics. For example, it is often critical that an oscillator output have a duty cycle that is as close to exactly 50% as possible. The duty cycle of an oscillator output may be defined as the duration in which the output is “high” (e.g., has a voltage at or proximate to its maximum operating voltage) divided by the period of the oscillator output signal (e.g., the inverse of the oscillator output frequency). Failure to maintain an exact or near-exact 50% duty cycle may cause spurious content (e.g., undesired harmonics) to appear in an oscillator or elsewhere in a receiver, transmitter, or transceiver of which it is a part.
In addition, it is often desired that once powered on, an oscillator begin operating as quickly as possible. One method to ensure fast startup is to provide a high bias voltage to one or more components of the oscillator. However, such high bias voltage may cause stress in a crystal resonator of the oscillator, which may lead to damage to the crystal resonator or undesired operation.